Butterfly valves are in common usage for controlling the flow of various fluids, i.e., liquid or gas, streams. Butterfly valves are used to throttle fluid flow and for on/off applications. A typical valve assembly of this type includes a body having a passage extending through it and a butterfly valve vane pivotally mounted within the body. The butterfly vane is characteristically in the form of a disc.
When fluid passes through a partially open butterfly valve, the fluid undergoes a significant pressure drop. One of the basic problems for butterfly valves is that this pressure drop tends to cause cavitation and consequent cavitation-induced damage in liquid service and noise in gas service.
In an attempt to solve these problems, it has been proposed to utilize a diffuser. A diffuser is a perforated member that increases the restriction near the valve opening and breaks the fluid stream into multiple jets. This has a positive effect on the cavitation and noise problems.
Generally, however, diffusers are either built into a particular valve or constructed to be applicable to only a single manufacturer's valve. For example, one such diffuser is integrally incorporated into the valve element such that it is not possible to utilize this diffuser in other valves. In addition, this diffuser has little or no affect on the ability of the valve element to provide improved flow control near the fully open position.
In addition, butterfly valves are generally not good control valves near the closed or fully opened positions of the valve. Specifically, near the closed position, small changes in the angle of the valve disc results in a relatively large change in flow, and in the nearly fully opened position, a relatively large change in the angle of the valve disc is required to produce even a relatively small change in flow.
For this reason, butterfly valves are currently used mainly for on/off purposes or for light controlling purposes. Since butterfly valves are notoriously uncontrollable at the ends of their actuation ranges, butterfly valves are not used in situations where accurate control is necessary. In situations that require accurate control, globe or segmented ball control valves are typically used. However, globe and segmented ball control valves are typically much larger and more expensive than butterfly valves. Thus, the ability to use a butterfly valve instead of either a globe or segmented ball control valve would be particularly desirable.
Although butterfly valves that include some type of an integral diffuser, or a separate diffuser element positioned at either the inlet or outlet, or both, resulting in the butterfly valve having better flow control characteristics than a butterfly valve without any type of integral diffuser, or separate diffuser element(s), it is desirable to provide a system for providing additional control capabilities of butterfly valves.